The Collaborative International Dictionary of English v.0.48:

Diploma \Di*plo"ma\, n.; pl. Diplomas. [L., fr. Gr. ?, fr. ?
   to double, fr. diplo`os twofold. See Double.]
   A letter or writing, usually under seal, conferring some
   privilege, honor, or power; a document bearing record of a
   degree conferred by a literary society or educational
   institution.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diploma
    n 1: a document certifying the successful completion of a course
         of study [syn: diploma, sheepskin]